doc: clarify buffer class

Fixes: https://github.com/nodejs/node/issues/6891
PR-URL: https://github.com/nodejs/node/pull/6914


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>